The present invention relates generally to systems used to protect structures from damage caused by the addition of external forces. More specifically, the present invention pertains to aseismic foundations used to isolate structures from destructive forces caused by earthquakes and strong winds. The present invention is particularly, though not exclusively, useful as a means for isolating a structure from the forces caused by seismic activity and high winds.
Each year many earthquakes occur around the world resulting in catastrophic loss of life and devastating property damage. In areas along active fault lines, it is important that structures be designed to resist the damage caused by earthquakes. With traditional construction methods, structures are rigidly attached to the earth""s surface.
When an earthquake occurs, the vibration of the earth is transferred directly to a structure""s foundation, which, in turn, transmits the vibration to the structure. This can result in enormous shear forces which may cause irreparable structural damage to the structure. Moreover, if a structure is subjected to high winds in conjunction with seismic forces, the catastrophic results may be even greater.
Consequently, numerous foundation systems designed to be xe2x80x9cearthquake proofxe2x80x9d have been invented. These xe2x80x9cearthquake proofxe2x80x9d foundation systems may be classified as antiseismic or aseismic. The antiseismic class involves foundations which are designed to withstand the great forces caused by earthquakes. These antiseismic systems are bigger and stronger than typical foundation systems and do not allow any motion of the structure relative to its foundation. These systems are not very effective in preventing damage to a structure because the destructive forces are still transmitted through the foundation to the structure.
The aseismic class involves foundations which are designed to isolate the structure from these destructive forces. Aseismic foundations are generally flexible and allow relative motion between the structure and the earth. During an earthquake, this relative motion keeps the structure from being subjected to the destructive forces normally associated with earthquakes.
One such invention, as disclosed in U.S. Pat. No. 5,181,356, which issued in 1993 to Sul for an invention entitled xe2x80x9cEarthquake Resistant Building Support Systemxe2x80x9d (the xe2x80x9c""356 patentxe2x80x9d), includes a pedestal which rests on a slab. In between the pedestal and the slab is an xe2x80x9cOxe2x80x9d ring or a sheet of metal to reduce the friction between the pedestal and the slab. The pedestal is fixed to the structure support and moves relative to the slab when the earth shifts back and forth during an earthquake.
The device of the ""356 patent is limited with respect to the amount of horizontal motion that it will allow. Moreover, the xe2x80x9cOxe2x80x9d ring or the sheet of metal sandwiched between the pedestal and the slab does not appear to reduce friction sufficiently to allow any motion at all. On the other hand, if either configuration reduces friction enough to allow some motion, the xe2x80x9cOxe2x80x9d ring or the metal sheet would be subjected to excessive wear and tear and would be nearly impossible or difficult to replace.
Another such invention, as disclosed in U.S. Pat. No. 4,771,581, which issued in 1988 to Nill for an invention entitled xe2x80x9cFluid Support System For Building Structuresxe2x80x9d (the xe2x80x9c""581 patentxe2x80x9d), includes the use of a liquid support system for separating a structure from its foundation. The device of the ""581 patent includes the use of elongated conduits filled with an incompressible liquid to absorb vertical movement of the foundation wall caused by expanding or contracting soils. This device does not appear to allow any horizontal motion usually associated with earthquakes. Moreover, the device of the ""581 patent is unable to account for and provide relief from the forces caused by strong winds in conjunction with seismic activity.
A third aseismic foundation, as disclosed in U.S. Pat. No. 4,766,706, which issued in 1988 to Caspe for an invention entitled xe2x80x9cEarthquake Protection System For Structuresxe2x80x9d (the xe2x80x9c""706 patentxe2x80x9d), includes the use of a layer of plates to allow relative motion between the structure and the foundation. One plate is fixed to the top of the foundation, one plate is fixed to the underside of the structure support, and a third plate is sandwiched between these two plates.
During an earthquake, the plates are intended to slide with respect to each other in the horizontal direction. The device of the ""706 patent is very complex and would likely be very expensive to build and maintain. Moreover, the device of the ""706 patent does not allow any pivotal motion at the structure support, which would account for the situation that occurs when the foundation is moved out of vertical by buckling earth.
Accordingly, it is the object of the present invention to provide an aseismic foundation which will isolate a structure from seismic forces by allowing relative motion between a structure support system and its foundation in the horizontal direction. It is another object of the present invention to provide an aseismic foundation that will allow the foundation to move pivotally with respect to a structure support pillar. It is another object of the present invention to provide an aseismic foundation which will prevent catastrophic damage to a structure when subjected to seismic activity. It is another object of the present invention to provide an aseismic foundation which will prevent catastrophic damage to a structure when subject to high winds in conjunction with seismic activity. It is another object of the present invention to provide an aseismic foundation which will be relatively easy to maintain. It is another object of the present invention to provide an aseismic foundation which will last the lifetime of the structure. It is yet another object of the present invention to provide an aseismic foundation which will be relatively easy to manufacture, relatively easy to install, and relatively cost effective.
The Improved Aseismic System of the present invention includes five major components which may be included in the construction of nearly any new structure erected in a seismically active area. These five components will drastically reduce, if not eliminate, the transmission of the seismic forces to these structures.
The first major component of the Improved Aseismic System of the present invention is the double-action hydraulic ram assembly. The double-action hydraulic ram assemblies are arranged under the base of the structure in a pattern reflecting the perimeter of the structure. Thus, a structure with a square perimeter would have four double-action ram assemblies arranged in a square its base.
Each double-action hydraulic ram assembly includes a large cylindrical housing encasing a hydraulic ram with a vertical piston centered axially on the ram. On each side of the piston is a hydraulic ram fluid chamber filled with an incompressible fluid. During an earthquake, as the tremors cause the earth to shift back and forth, the structure will slide on the rams depending on the direction of the motion caused by the tremor.
As the structure slides, the incompressible fluid is forced out of the hydraulic ram fluid chamber on the trailing side of the structure and into the next major component of the Improved Aseismic System of the present invention, the gas dampener. The gas dampener counteracts the motion of the structure along the double-action hydraulic ram assembly and returns the structure to its original position. Each chamber of the double-action hydraulic ram assembly includes a separate gas dampener.
The Improved Aseismic System of the present invention includes eight gas dampeners. Each gas dampener has three concentric pistons in a single cylinder separated by gases having different pressures. The bottom surface of the first piston in the gas dampener contacts the incompressible fluid shared with the corresponding hydraulic ram fluid chamber. As the fluid is forced into the gas dampener, the first piston is forced upward.
In between the first piston and the second piston is a compressible gas. As the first piston is forced upward, it compresses the first gas until it reaches the same pressure as the second gas. When the first gas pressure equals the second gas pressure, the first and second pistons will be forced upward in unison compressing the third gas until it reaches a pressure equal to the pressure of the first gas and the second gas. When the third gas pressure equals the first and second gas pressure, the third piston will also be forced upwards in unison with the first and second pistons.
Above the third piston is a static chamber with a gas valve that allows the third gas to pass into the static chamber. The top of the third piston is equipped with a spring loaded stem that is aligned with the gas valve in the static chamber. As the top of the third piston approaches the static gas chamber, the spring loaded stem engages the gas valve, closes the gas valve, and ceases the flow of gas into the static gas chamber.
With the gas valve closed, the third gas pressure will increase rapidly as the fluid continues to be forced into the gas dampener by the double-action hydraulic ram assembly. The rapid increase in gas pressure will counteract the effects of increased motion along the double-action piston and will prevent the structure from crashing into the end the ram or the foundation.
The third major component is the flow control assembly. The Improved Aseismic System of the present invention includes eight flow control assemblies. A flow control assembly is installed between each hydraulic ram fluid chamber and its corresponding gas dampener. The flow control assembly regulates the flow of the incompressible fluid between the hydraulic ram fluid chamber and corresponding gas dampener.
As the structure with the Improved Aseismic System is subjected to a load due to an earthquake, the flow control assembly allows the incompressible fluid to flow freely between the hydraulic ram fluid chamber and the gas dampener. However, when the structure is subjected to wind forces in addition to an earthquake load, the fluid flow control assembly responds appropriately depending on the direction of the wind in relation to the motion caused by the tremors.
The fourth major component of the Improved Aseismic System of the present invention is the pressure generator. The pressure generator includes a sail attached to a large arm which is balanced by a counter weight. The arm includes a centrally located ball which fits into a socket to allow the sail and arm to spin three hundred and sixty degrees (360xc2x0 ) and bob up and down under the power of the wind. Attached to the arm is a plunger actuator connected to one or more plungers installed in corresponding cylinders which are, in turn, connected to corresponding fluid flow control assemblies via a pipe containing incompressible fluid.
As the plunger actuator moves, it causes a change in pressure in the incompressible fluid contained in the pipes between the cylinders and the flow control assemblies. An initial change in pressure will close a valve in the flow control assembly and block the flow of fluid between the hydraulic ram fluid chamber and the gas dampener. Any further pressure change will result in the injection of more incompressible fluid into the lower fluid chamber within the flow control assembly.
The fifth, and final, major component of the Improved Aseismic System of the present invention is the seismic filter. The seismic filter consists of a fluid confined between two plates. One plate is affixed to the bottom of a structure support pillar and the other plate is affixed to the foundation. The confined fluid keeps the two plates from contacting each other and allows for relative motion between the plates. The foundation plate further includes a convex surface and a concave surface separated from each other by a lubricant. This configuration allows for slight pivotal motion at the base of the structure if the foundation is shaken out of level during an earthquake.
The invention as described above overcomes many of the disadvantages of the previous aseismic foundation systems. The present invention provides an aseismic foundation system that allows relative motion between a structure support system and foundation in any horizontal direction. The present invention also prevents catastrophic damage to a structure when subjected to seismic forces. Moreover, the present invention prevents catastrophic damage to a structure which is subject to high winds in conjunction to seismic forces. Finally, the present invention is relatively easy to install, and relatively easy to maintain.